1. The mating system of Mediterranean fruit flies (Ceratitis capitata, Diptera: Tephritidae), is based on male leks that form on the foliage of trees. Following observations that not all males participate in leks, the hypotheses that (a) small males are absent from leks and (b) males with inadequate nutrient reserves are unable to participate in leks were examined.

2. The size and weight of lekking males (n = 183) and resting males captured at the same time in the vicinity of leks (n = 148) were established. In addition, using biochemical techniques, the amounts of sugar, glycogen, lipid and protein in each individual were established quantitatively.

3. There was no significant size difference between lekking and resting males. However, lekking males were significantly heavier and contained significantly more sugars and protein than resting males.

4. In conclusion, leks are exclusive, and only males with adequate nutritional reserves may join. Thus, in this species, reproductive success is closely linked to foraging success.